A typical automotive air-conditioner for controlling a temperature in an interior has a hot-water heating system. The system includes a closed circuit in which a heat transporting medium or water is circulated. During circulation, heat is provided to the water and then transmitted to air to be fed into the interior.
Referring to FIG. 7, there is shown a schematic block diagram of the prior art hot-water heating system, generally indicated by reference numeral 10, of the automotive air-conditioner. The hot-water heating system 10 has a closed passage 12 for circulating a heating medium or water W. The passage 12 includes a heater unit 14 for heating the water W, a circulation pump 16 for circulating the water W in the closed passage 12, a heat exchanger 18 for exchanging heat between the heated water W and air to be fed into the interior.
The heater unit 14 is equipped with an electric heater 20. The heater 20 is connected through a switching means or relay 22 for controlling a supply of an electric power to the heater 20 and then a fuse 24 designed to blow when it is heated up to a certain high temperature with a DC power source 26. The relay 22 includes a contact 28 that takes a first position in which it permits the heater 20 to be energized and a second position in which it prohibits the heater 20 to be energized and a coil 30 for moving the contact 28 between the first and second positions. The coil 30 is connected with a controller 32 which is in turn connected with a temperature sensor 34 for sensing a temperature of the heated water W in the circuit and a plurality of inputs mounted on an operation panel 36 in the interior. The inputs, among others, include a switch 38 for turning on and off the air-conditioner and a switch 40 or volume for setting an air-conditioning temperature of an ambient in the interior to be air-conditioned. Further, the coil 30 is connected with another DC power source 42.
In operation of the automotive air-conditioner, the water W is circulated in the passage 12 in the direction indicated by arrows by the circulation pump 16. In the circulation, the water W is heated at the heater 20. The heated water W is then fed into the heat exchanger 18 where heat of the water W is transmitted to air to be fed into the interior.
The temperature of the heated water W is controlled based on a comparison of between the air-conditioning temperature set by the switch 40 and the water temperature detected at the temperature sensor 34. For example, the controller 32 controls the relay 22 to turn on and off the electric power from the DC power supply 26 to the heater 20.
According to the air-conditioner with the hot-water heating system, once a contact 28 of the relay 22 is fused to lead a short-circuit of the circuit, the heater 20 is out of control, resulting in an overheating of the heater 20. At worst, the overheating, unless eliminated, would provide serious problems, such as breakdown of the heater 20. To prevent this, the system 10 is designed so that the fuse 24 detects the overheating and then disconnects the circuit.
Normally, because the hot-water heating system is positioned out of the interior and, therefore, subject to water or rain, the fuse 24 should be waterproofed. Disadvantageously, this results in a bulky and costly fuse unit. Also, the ambient temperature provides an adverse affect to the fuse 24, leading a great difficulty in a design of the fuse capable of working precisely.
Therefore, there has been a need for a technique capable of switching off the heating system of the air-conditioner without using any fuse to prevent the overheating when the relay is broken down.